Conventionally, there have been known various types of liquid image forming devices that perform development using a liquid developer consisting of a carrier liquid and toner dispersed therein (see e.g., Japanese Patent Application Laid-Open No. 7-209922, Japanese Patent Application Laid-Open No. 7-152254, or Japanese Patent Application Laid-Open No. 7-21935). Further, the applicant of this invention has proposed an image forming method in Japanese Patent Application No. 11-38447, in which a developer carrier having an elastic layer is brought into contact with a latent image carrier to form a nip part for development. In this image forming method, a thin layer of a liquid developer is formed on the developer carrier, and a carrier liquid and toner in the thin layer are electrostatically transferred to an image portion of a latent image on the latent image carrier that forms the developing nip part, where development is performed. On the other hand, toner is inhibited from adhering to a background portion (non-image portion) of the latent image carrier passing through the developing nip part but a slight amount of carrier liquid is allowed to migrate thereto.
Even if the toner adheres to the background portion, the toner can be moved to the developer carrier to be removed while the background portion is passing through the developing nip part.
However, in the method of moving the toner on the non-image portion to the developer carrier and removing it, the toner may adhere to the background portion (non-image portion) of the latent image carrier passing through the developing nip part and remain thereon as excess toner. Thus, image fog may occur due to the excess toner.
To solve the problem, the applicant of this invention has proposed a device provided with a removing member, that removes excess toner on the latent image carrier after development, located downstream the developing nip part in a direction in which the surface of the latent image carrier moves in order to form high quality images by preventing image fog due to excess toner (see Japanese Patent Application No. 2000-42582).
Further, in Japanese Patent Application No. 2000-42582, the applicant of this invention has also proposed provision of a cleaning unit for cleaning the surface of the removing member in order to maintain removal performance of the removing member that removes the excess toner on the latent image carrier by coming into contact with the surface of the latent image carrier.
Conventionally, there has been known a liquid developing device that applies a liquid developer consisting of a carrier liquid and toner dispersed therein to an elastic developing roller, brings the developing roller by pressure into contact with a photoreceptor as a latent image carrier where an electrostatic latent image is formed, develops the electrostatic latent image using the liquid developer applied to the developing roller, and removes the toner adhering to the background of the latent image carrier using a sweep roller.
The conventional liquid developing device forms a predetermined contact width (nip) by bringing the developing roller into contact with the photoreceptor and pressuring the roller against the photoreceptor, moves the toner dispersed in the liquid developer adhering to the developing roller to the photoreceptor, and adhere the toner to an electrostatic latent image formed on the photoreceptor to visualize the electrostatic latent image thereon.
In Japanese Patent Application Laid-Open No. 2000-242088, the applicant of this invention has proposed an image forming method of forming a nip part by bringing a developer carrier having an elastic layer into contact with a latent image carrier. In this image forming method, a thin layer of a liquid developer is formed on the developer carrier, and the carrier liquid and toner in the thin layer are electrostatically transferred to an image portion of the latent image on the latent image carrier to perform development. On the other hand, the toner is inhibited from adhering to the background portion (non-image portion) on the latent image carrier passing through the nip part but a small amount of carrier liquid is allowed to migrate toward the background. Even if the toner adheres to the background, the toner can be removed by being transferred to the developer carrier while the background is passing through the nip part.
However, in a structure in which a removing member for removing excess toner from the latent image carrier is provided, if an area where the toner is removed by the removing member is smaller relative to an area where the liquid developer is applied to the latent image carrier, the excess toner may not fully be removed. Resultantly, an excess toner remaining area may occur on the latent image carrier.
An area where the liquid developer is applied to the surface of the latent image carrier covers an area where an image becomes effective through development (hereafter called “effective image area”), therefore, the area is generally set to be slightly broader than the effective image area. It is generally thought that an area where excess toner is removed by the removing member also covers the effective image area and is therefore set to be slightly broader than this effective image area.
In this case, even if the excess toner removal area is made broader than the effective image area, it may be narrower than the liquid developer applied area. Therefore, the toner outside the excess toner removal area and within the liquid developer applied area is not removed to remain on the surface of the latent image carrier.
If the excess toner is left on the latent image carrier, a transfer medium for transferring the image on the latent image carrier therefrom, may be soiled with the excess toner and so are the peripheral members.
Image fog due to the excess toner is found more noticeable particularly when a highly viscous and highly concentrated liquid developer is used. Consequently, necessity of the removing member is increased.
On the other hand, the conventional liquid developing device is so constructed that the developing roller is always pressurized against and in contact with the photoreceptor. Therefore, if the developing roller is in contact with the photoreceptor and left standing as it is for long time, distortion may occur in the developing roller. Further, the surface of the photoreceptor in contact with the developing roller may be soiled.
Therefore, it is conceivable that the developing roller is separated from the photoreceptor when the liquid developing device is not in use. However, if the photoreceptor and the developing roller are brought into contact with or separated from each other, the surface of the developing roller and the surface of the photoreceptor may be damaged or scratched due to a difference between a rotating speed of the photoreceptor and that of the developing roller.
Abnormal discharge may occur immediately before the developing roller comes into contact with the photoreceptor or immediately after the developing roller separates from the photoreceptor due to a potential difference between the surface of the developing roller and that of the photoreceptor, thereby the surface of the developing roller or the surface of the photoreceptor may be damaged.
Further, the toner adheres to the photoreceptor, which causes the toner consumption to increase.
In the image forming method proposed in Japanese Patent Application Laid-Open No. 2000-242088 as mentioned above, the toner may adhere to the background portion (non-image portion) on the latent image carrier passing through the nip part and remain as excess toner. In this case, image fog due to this excess toner may occur. Further, the carrier liquid adhering to the image portion and non-image portion may be unnecessarily consumed.